1. Field of the Invention
The present invention relates to a digital photo-detector, and more specifically, to a thin film transistor array substrate for a digital photo-detector having reduced leakage current and noise.
2. Discussion of the Related Art
An X-ray is a short wavelength radiation that easily passes through a subject, and transmittance of X-rays depends on a density of the subject. That is, inner characteristics of the subject may be indirectly observed through amount of X-rays passing through the subject.
An X-ray detector is a device that detects an amount of X-ray passing through the subject. The X-ray detector detects transmittance of X-ray and displays the inner characteristics of the subject on a display device. The X-ray detector may be generally used as a medical inspector, a non-destructive inspector, or the like.
In recent years, a digital X-ray detector using digital radiography without using a film is widely used as an X-ray detector. Each cell of a thin film transistor array for a digital X-ray detector includes a photo-diode (PIN diode) that receives X-rays, converts the X-rays into visible light, and converts the visible light into an electric signal. A thin film transistor that is formed under the photo-diode outputs the electric signal from the photodiode to a data line.
FIG. 1 is a view illustrating a configuration of a general digital X-ray detector 100. As shown in FIG. 1, the general digital X-ray detector 100 includes a thin film transistor array substrate 110, a bias supplier 120, a gate driver 130, a readout integrated circuit 150, a timing controller 180, and a power-supply voltage supplier 190. The readout integrated circuit 150 includes a signal detector 160 and a multiplexer 170.
The thin film transistor array substrate 110 detects an X-ray emitted from an energy source, photo-electrically converts the detected X-ray into an electric signal, and outputs an electric signal. The thin film transistor array substrate 110 includes a plurality of gate lines (GL), a plurality of data lines (DL) arranged in a vertical direction to the gate lines (GL) to define respective cell regions, and a plurality of photosensitive pixels (P) arranged in a matrix form in respective cell regions by the gate lines and the data lines.
Each photosensitive pixel (P) includes a photodiode (PD) that detects an X-ray and outputs a detection signal, e.g., photo-detection voltage, and at least one switching device for outputting the detection signal from the photodiode (PD) to the data line (DL) in response to a gate pulse. For example, the switching device is a transistor. Hereinafter, a configuration in which the switching device is a transistor will be described.
The photodiode (PD) senses an X-ray emitted from an energy source and outputs the sensed signal as a detection signal. The photodiode (PD) is a device that converts incident light into an electrical detection signal through photoelectric effect and is, for example, a PIN diode having a p-type semiconductor layer, an intrinsic (I) semiconductor layer and an n-type semiconductor layer laminated in this order.
The bias supplier 120 applies a driving voltage through a plurality of bias lines (BL). The bias supplier 120 may apply a predetermined voltage to the photodiode (PD), or selectively apply a reverse bias or a forward bias thereto.
The gate driver 130 sequentially applies gate pulses having a gate-on voltage level through the gate lines (GL). In addition, the gate driver 130 may apply reset pulses having a gate-on voltage level to a plurality of reset lines (RL). The gate-on voltage level is a voltage level that turns on transistors of the photosensitive pixels (P). The transistors of the photosensitive pixels (P) may be turned on in response to the gate pulse or the reset pulse.
The detection signal output from the photodiode (PD) in response to the gate pulse is input through the data lines (DL) to the readout integrated circuit 150. The gate driver 130 may be mounted in an IC form at one side of the thin film transistor array substrate 110, or formed on a substrate, such as the thin film transistor array substrate 110, through a thin film process.
The readout integrated circuit 150 reads out the detection signal output from the turned-on transistor in response to the gate pulse. The readout integrated circuit 150 read outs a detection signal output from the photosensitive pixel P in an offset readout region to read out an offset image and in an X-ray readout region to read out a detection signal after X-ray exposure.
The readout integrated circuit 150 may include a signal detector 160 and a multiplexer 170.
The signal detector 160 includes a plurality of amplification units that correspond to the data lines (DL) one to one. Each amplification unit includes an amplifier (OP), a capacitor (CP), and a reset device (SW).
The timing controller 180 generates a start signal (STV), a clock signal (CPV) or the like, and outputs the same to the gate driver 130 in order to control operation of the gate driver 130. In addition, the timing controller 180 generates a readout control signal (ROC), a readout clock signal (CLK) or the like, and outputs the same to readout integrated circuit 150 in order to control operation of the readout integrated circuit 150. The gate driver 130 and the readout integrated circuit 150 may be operated using separate clock signals. The power-supply voltage supplier 190 supplies a power-supply voltage to the photosensitive pixels (P) through the power-supply voltage lines (VDD).
A unit cell structure of the thin film transistor array for the general digital X-ray detector will be described below.
FIG. 2 is a view illustrating a circuit configuration of a unit cell of a related art thin film transistor array substrate for a digital X-ray detector, FIG. 3 is a plan view illustrating the unit cell of the related art thin film transistor array substrate for a digital X-ray detector, and FIG. 4 is a sectional view taken along the line I-I′ of the unit cell of the related art thin film transistor array substrate for a digital X-ray detector.
As shown in FIGS. 2 and 3, unit cells of the related art thin film transistor array substrate for a digital X-ray detector is defined by a plurality of gate lines (GL) to supply a scan signal, a plurality of data lines (DL) arranged in a direction vertical to the gate lines (GL) to output a data, a photodiode (e.g., a PIN-diode) formed in respective cell regions defined by the gate lines and the data lines to perform photoelectric conversion, a thin film transistor (TFT) formed at each of intersections between the gate lines (GL) and the data lines (DL) to turn on according to the scan signal of the gate lines and output the photoelectric conversion signal from the photodiode to the data lines, and a plurality of bias lines (BL) to apply a bias voltage to the respective photodiodes. As shown in FIG. 3, the data lines (DL) and the bias lines (BL) are formed parallel to each other between adjacent cells.
The cross-sectional structure of such a unit cell will be described below. As shown in FIG. 4, the gate line (represented by “GL” in FIGS. 2 and 3) and a gate electrode 2 protruding from the gate line are formed on a substrate 1, and a gate insulating film 3 is formed over the entire surface of the substrate having the gate electrode 2.
In addition, an active layer 5 is formed on the gate insulating film 3 over the gate electrode 2, and a drain electrode 4a and a source electrode 4b are formed at both sides of the active layer 5 to constitute the thin film transistor. A first interlayer insulating film 7 is formed on the entire surface of the substrate including the drain electrode 4a and the source electrode 4b, and the first interlayer insulating film 7 arranged on the source electrode 4b of the thin film transistor is selectively removed to form a first contact hole 6.
A first electrode 8 of the photodiode is formed on the first interlayer insulating film 7 such that it is connected to the source electrode 4b of the thin film transistor through the first contact hole 6. A semiconductor layer 9 having a p-type semiconductor layer, an intrinsic semiconductor layer and an n-type semiconductor layer is formed on the first electrode 8, and a second electrode 10 of the photodiode is formed on the semiconductor layer 9.
A second interlayer insulating film 11 is formed on an entire surface of the first interlayer insulating film 7 including the second electrode 10 of the photodiode, the first and second interlayer insulating films 7 and 11 arranged on the drain electrode 4a of the thin film transistor are selectively removed to form a second contact hole 16, and the second interlayer insulating film 11 arranged over the second electrode 10 of the photodiode is selectively removed to form a third contact hole 17.
On the second interlayer insulating film 11, a plurality of data lines 12 (DL) connected to the drain electrode 4a of the thin film transistor through the second contact hole 16 are formed. A light-shielding layer 13 is formed over a channel region of the thin film transistor, and a plurality of bias lines 14 (BL) connected to the second electrode 10 of the photodiode through the third contact hole 17 are formed. In addition, a protective film 15 is formed over the entire surface of the substrate.
Here, because a channel region of the thin film transistor (e.g., an active region 5 between the source electrode 4b and the drain electrode 4a) is exposed when seen from a direction in which an X-ray is transmitted, the thin film transistor generates light leakage current when X-rays are incident. Accordingly, to solve this problem, the light-shielding layer 13 is formed over the channel region of the thin film transistor. As shown in FIGS. 3 and 4, a bias voltage is applied to the light-shielding layer 13 through the bias line (BL) electrically connected thereto.
The related art thin film transistor array substrate for a digital X-ray detector having this configuration operates as follows.
When X-rays irradiates the device, current flows in the photodiode according to a light dose corresponding to the intensity of the X-rays, and when a scan signal (a gate high voltage) is applied to the gate line, the thin film transistor turns on and outputs an optical signal through the data line.
However, the related art thin film transistor array substrate for a digital X-ray detector having this configuration has the following problems.
A bias voltage (e.g., −4V) should be applied to the second electrode of the photodiode so that the digital X-ray detector can detect photocurrent corresponding to the X-ray intensity. Accordingly, the bias voltage is applied to the second electrode of the photodiode through the bias line. In addition, the bias voltage is applied to the light-shielding layer.
When the first interlayer insulating film and the second interlayer insulating film have complete insulating properties, a back channel is not formed in the thin film transistor due to the bias voltage applied to the light-shielding layer.
However, when processes are changed or the bias voltage is increased, a back channel is formed on the thin film transistor due to the light-shielding layer, a current pass is formed in the back channel of the thin film transistor, and a back channel leakage current of the thin film transistor is generated. However, a scan pulse is not supplied to the gate line in a case excluding a gate-selection region.
When a scan pulse is supplied to the gate line (gate selection region), a photocurrent of the photodiode is output to the data line through a normal channel (e.g., a channel passing through the active region) of the thin film transistor, but a current pass is formed on the back channel of the thin film transistor by the light-shielding layer, and a noise signal is thereby output to the data line.
The problems may be demonstrated through experiments. FIG. 5 is a graph showing an operation of a related art thin film transistor according to bias voltage.
For example, an initial condition is a current value of the thin film transistor in a state in which the same bias voltage (e.g., −4V) as in operation of the digital X-ray detector is applied to the bias line. Dot lines represent current values measured after bias voltages of 0 to 20V, and of 0 to −20V are applied to the bias line and a stress is applied to the back channel.
It may be ideal that the case of the initial condition and the case in which positive and negative stresses are applied to the bias electrode exhibit the same current values. However, as can be seen from FIG. 5, the initial state and the stress-applied state have different current properties. The reason for this phenomenon is that charges remain after the stress is applied to the back channel.